The background of the invention is related to injection-technical problems associated with fluid injection, preferably water injection, into a reservoir via a well. Such injection-technical problems are particularly prevalent when injecting from a horizontal well. These problems often result in downstream reservoir-technical and/or production-technical problems.
During fluid injection, the injection fluid flows out radially through openings or perforations in the injection string. Depending on the nature of the reservoir rock in question, the injection string is either fixed through cementation or disposed loosely in a borehole through the reservoir. The injection string may also be provided with filters, or so-called sand screens, preventing formation particles from flowing back into the injection string during a temporary break in the injection.
When the injection fluid is flowing through the injection string, the fluid is subjected to flow friction, which results in a frictional pressure loss, particularly when flowing through a horizontal section of an injection string. This pressure loss normally exhibits a non-linear and greatly increasing pressure loss progression along the injection string. Thus the outflow rate of the injection fluid to the reservoir will also be non-linear and greatly decreasing in the downstream direction of the injection string. At any position along a horizontal injection string, for example, the driving pressure difference (differential pressure) between the fluid pressure within the injection string and the fluid pressure within the reservoir rock therefore will exhibit a non-linear and greatly decreasing pressure progression. Thereby, the radial outflow rate of the injection fluid per unit of horizontal length will be substantially greater at the upstream “heel” of the horizontal section than that of the downstream “toe” of the well, and the fluid injection rate along the injection string thereby becomes irregular and decreasing. This causes substantially larger amounts of fluid being pumped into the reservoir at the “heel” of the well than that of its “toe”. Thereby, the injection fluid will flow out of the horizontal section of the well and spread out within the reservoir as an irregular, non-uniform (inhomogeneous) and partly unpredictable flood front, inasmuch as the flood front drives reservoir fluids towards one or more production wells. Normally, such an irregular, non-uniform and partially unpredictable flood front is unfavourable with respect to achieving optimal recovery of the fluids of the reservoir.
An uneven injection rate may also occur as a result of inhomogeneity within the reservoir. The part of the reservoir having the highest permeability will receive most fluid. This creates an irregular flood front, and the fluid injection thus becomes non-optimal with respect to downstream recovery from production wells.
To prevent or reduce such an irregular injection rate profile along the injection string, it is desirable to pump the injection fluid into the reservoir at a predictable radial outflow rate per unit of length of a horizontal injection string, for example. Normally, it is desirable to pump the injection fluid at equal or approximately equal radial outflow rate per unit of length of the injection string. Thereby, a uniform and relatively straight-line flood front is achieved, moving through the reservoir and pushing the reservoir fluid in front of it. This may be achieved by appropriately adjusting, and thereby controlling, the energy loss (pressure loss) of the injection fluid as it flows radially out from the injection string and into the reservoir. The energy loss is adjusted relative to the ambient pressure conditions of the string and of the reservoir, and also to the reservoir-technical properties at the outflow position/-zone in question.
In connection with a horizontal well, it may also be desirable to create a flood front having a geometric shape that, for example, is curvilinear, arched or askew. Thereby, it is possible for a reservoir to better adjust, control or shape the flood front relative to the specific reservoir conditions and properties, and relative to other well locations. Such adaptations, however, are difficult to carry out by means of known injection methods and equipment.
An irregular, non-uniform and partly unpredictable flood front may also emanate from a non-horizontal well. The above-mentioned fluid injection problems therefore are relevant to non-horizontal wells, too.
Principally, this invention seeks to remove or limit this unpredictability and lack of control of the injection flow, this resulting in a better shape and movement of the fluid front within the reservoir.